The springs used to support automobile trunk lids, hoods, and the like, especially the hatch-back trunk lid and station wagon tailgates, are often of the gas spring variety. A gas spring is essentially a sealed cylinder containing a gas under high pressure and having a piston rod extending from one end of the cylinder. Typically, nitrogen gas having a pressure of approximately 6900 kPa (1000 psi) is used in the cylinder. The spring force results from the pressure of the gas acting on a cross-sectional area equal to that of the rod within the cylinder and urging the rod outwardly. When the rod is pushed into the cylinder, as when the hatchback trunk lid is closed, the rod displaces a certain volume within the cylinder which was previously occupied by the gas. Since the total volume within the cylinder is fixed, the remaining volume available to the gas decreases, resulting in an increase in the pressure of the gas. Thus, the force acting to move the rod outward increases. In conventional gas springs, a piston-like structure may be attached to the rod inside of the cylinder and used for damping and limiting the extend of motion of the rod. Since the gas pressure is normally equal both sides of the piston, it produces little, if any, force on the rod.
Ideally, the pressure of the gas should be sufficient to generate a force large enough to move the piston rod outwardly from the cylinder and lift the trunk lid or the like to which it is attached. The gas pressure should also be low enough when the rod is completely extended and the trunk lid or the like is raised to enable a person easily to move the rod into the cylinder when the trunk lid is being closed. A drawback arising from the use of a gas spring is that the pressure of any gas in a fixed volume changes as the temperature of the gas changes. For an ideal gas, which nitrogen resembles, the pressure is directly proportional to the absolute temperature of the gas. The change in gas pressure due to change in temperature can cause considerable problems when gas springs are used in automobiles, which commonly are exposed to ambient temperatures ranging from below -18.degree. C. (0.degree. F.) to above 40.degree. C. (100.degree. F.).
When the ambient temperature is low, the pressure of the gas inside the cylinder is low, resulting in insufficient force to urge the rod outwardly to lift the weight of the trunk lid or hold it up after it is lifted. When the ambient temperature is high, the pressure of the gas inside the cylinder is high, resulting in a large force urging the rod out of the cylinder, a situation which may cause a trunk lid connected to the rod to be raised undesirably rapidly. Furthermore, when the ambient temperature is high, the gas pressure inside the cylinder is large when the rod is completely extended, making it difficult to move the rod into the cylinder when it is desired to close the trunk lid.
It is an object of the present invention to provide a gas spring mechanism in which the sensitivity of the spring force to temperature variation is reduced to an acceptably low level.